designing and wrirting

Programming Logic and Design

Fourth Edition, Introductory

Chapter 4Designing and Writing a Complete Program

Programming Logic and Design, Introductory, Fourth Edition 2

Objectives

• Plan the mainline logic for a complete program

• Describe typical housekeeping tasks

• Describe tasks typically performed in the main loop of a program

• Describe tasks performed in the end-of-job module

• Understand the need for good program design


Objectives (continued)

• Appreciated the advantages of storing program components in separate files

• Select superior variable and module names

• Design clear module statements

• Understand the need for maintaining good programming habits


Understanding the Mainline Logical Flow Through a Program

• Understand what the goals are– Ask the user to clarify if necessary


Understanding the Mainline Logical Flow Through a Program (continued)

• Ensure you have all the data required to produce the desired output



• Understand the big picture first



• Procedural program: one procedure follows another from beginning to end

• Mainline logic has three distinct parts:– Housekeeping: steps to get ready– Main loop: instructions executed for every input record– End-of-job: steps taken at end of program

• Break the logic down into at least three modules



• Modularization of the program:– Keeps the job manageable– Allows multiple programmers to work simultaneously– Keeps the program structured


Housekeeping Tasks

• Housekeeping tasks: include all steps that occur at the beginning of the program– Declare variables– Open files– Perform one-time-only tasks such as printing

headings– Read the first input record


Declaring Variables

• Assign identifiers to memory locations

• Specify the name and data type

• Use meaningful names and follow standards

• Prefixes may be used to group related variables

• Declare a variable for each field in a data file


Declaring Variables (continued)



• Group name: – Name for a group of associated variables– Can handle the entire group with a single instruction



• Initializing (or defining) the variable: providing an initial value

• Some languages provide default initial values

• Other languages leave variables with an unknown or garbage value

• Variables representing data fields in files do not need to be initialized



• Can use variables for report headings• Embed any required spaces

• Heading can be printed using these variables



• Every language provides methods for:– Advancing the paper to top of page– Printing single, double, or triple spaced lines

• Use annotation symbol to show variables


Opening Files

• Specify file name and path (location)

• Issue a file open command

• If no input file is opened, input may be accepted from the standard input device (e.g., keyboard)

• You must open both input and output files to be used, including printer output device

• If no output file is opened, standard output device (e.g., monitor) may be used


A One-Time-Only Task -- Printing Headings

• Printing headings for reports usually is done at beginning of the program


Reading the First Input Record

• Reading the first input record is the last housekeeping task

• Interactive application: – Interacts with users via keyboard or mouse input– Program pauses when the read command is

executed until the user enters data• Delimiter: a character designated as a separator

between data values• Prompt: an output statement that asks the user to

enter specific data


Reading the First Input Record (continued)

• Interactive input:


Reading the First Input Record (continued)

• Input from a data file:

• Input from a data file using a group name:


Checking for the End of the File

• First task after housekeeping• For an interactive program, EOF may be determined

when:– User enters a predetermined sentinel value– User selects a screen option using a mouse

• For input from a file, the input device recognizes EOF• If no data in the file, EOF occurs on the first read• If there is data, each record is processed before the

next read occurs


Checking for End of File (continued)


Checking for End of File (continued)• Handling the report headings in a separate module:


Writing the Main Loop

• Each data record passes through the main loop once

• Inventory program main loop steps:1. Calculate the profit for an item

2. Print the item’s information on the report

3. Read the next inventory record


Writing the Main Loop (continued)

• Must declare additional variables for calculation results



• Detail lines are printed one line at a time:

• Calculations can be done within the print statement:

• Work variable (or work field): a variable used to temporarily hold a calculation


Performing End-of-Job Tasks

• End-of-job tasks may include:– Printing summaries or grand totals– Printing “End of Report” message– Closing any open files

• Footer line (or footer): end-of-job message line


Performing End-of-Job Tasks (continued)


Understanding the Need for Good Program Design

• Good design is:– Critical for very large programs– Needed to guarantee that components work together

properly• Well-designed program modules should work:

– As stand-alone modules – As part of larger systems


Storing Program Components in Separate Files

• Large programs may contain hundreds of variables and thousands of lines of code

• Manage lengthy programs by breaking into modules

• Many languages allow program components to be stored in separate files

• Storing components separately simplifies reuse

• Accessing modules from separate files is done with a statement like include, import, or copy


Storing Program Components in Separate Files (continued)


Storing Program Components in Separate Files (continued)

• Advantages of storing components separately:– Simplifies reuse– Can be provided in compiled form only, to hide details

• Implementation hiding: hiding details of how a program or module works


Selecting Variable and Module Names

• Using meaningful names:– Improves code readability– Is a form of self-documenting the program

• Use pronounceable names• Commonly used abbreviations are ok (e.g., SSN)• Avoid digits in a name to avoid confusing:

– Zeros and O’s– Ones and lowercase L’s


Designing Clear Module Statements

• Follow these rules:– Select good identifier names– Avoid confusing line breaks– Use temporary variables to clarify long statements– Use constants where appropriate


Avoiding Confusing Line Breaks

• Free-form coding allows programmer to decide where to break lines of code


Using Temporary Variables to Clarify Long Statements

• Use temporary variables to store intermediate results


Using Constants Where Appropriate

• Named constant: a constant whose value never changes during execution

• Advantages– Improves code readability– If the value changes later, there is only one place in

the code to make the change

• Usually written with all uppercase letters– ATHLETIC_FEE– TUITION_PER_CREDIT_HOUR


Using Constants Where Appropriate (continued)


Maintaining Good Programming Habits

• Program will be better written if you plan before you code

• Walk through program logic on paper before coding (desk-checking)

• Select good variable and module names for readability


Summary

• Three steps to designing a good program:– Understand the output that is required– Ensure you have the necessary input data– Plan the mainline logic

• Housekeeping tasks done at the beginning of the program: declaring variables, opening files, printing headings

• Main loop is controlled by EOF decision• Each data record passes through the main loop once


Summary (continued)

• End-of-job steps done at end of the program: printing summaries, closing files

• Good design becomes more critical as programs get larger

• Program components can be stored in separate files

• Select meaningful, pronounceable names

• Avoid confusing line breaks, use temporary variables, and use constants where appropriate

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